Existing data on hidden variation, much of it gathered in my lab, is anomalous in two respects: 1. Too few heterozygotes are observed; 2. Most alleles are very rare. And yet the variation is heritable. One hypothesis consistent with these results is that much of the variation reflects post-translational modification of proteins by other polymorphic loci. Recent data on XdH strongly supports this possibility. An extensive survey of "hidden" electrophoretic variation in Drosophila melanogaster is proposed, using a high-resolution analytic technique, gel sieving. For several selected loci, an attempt will be made by examining co-isogenic lines to locate all variants to the appropriate chromosome, and to map them. This will provide a direct determination of the proportion of the heritable variation resulting from second-site post-translational modification. Kinetic analysis will be carried out to assess the degree to which variation in shape entails functional differences.